In the main hereinafter the control of helicopters is discussed, but the teachings disclosed herein are relevant to aircraft generally.
For various reasons a pilot may decide to abort a landing approach or, in other words, initiate a "go-around". Under manual control the pilot pulls up on the collective stick to increase lift and, in a slow speed approach, may pitch the helicopter forward to increase air speed to facilitate lift. The most salient objective in a go-around is to clear obstacles, such as buildings, power lines or trees.
A flight director is responsive to various dynamic aircraft conditions and, in some instances, to signals originating from the landing field, and is operable to control the overall descent of the helicopter in a landing approach. It is known to provide a go-around mode in the flight director to initiate a go-around in response to a switch closure by the pilot.
The go-around mode has been implemented using an open loop approach to command the collective stick to a pre-determined position to establish a rate of climb. The rate of climb is related to the "power required" curve of the aircraft. For example, in a Sikorsky S-76 helicopter, the least amount of power is required to climb at 74 knots which is known as the "maximum rate of climb airspeed". Therefore it is known to establish closed-loop control over the airspeed to achieve a reasonable airspeed, such as 65 knots, while commanding the predetermined fixed amount of collective, and then optimizing the airspeed. When pitching nose-down to increase airspeed, much of the collective increase may be manifested in the airspeed axis rather than in the lift axis. The resulting rate of climb varies with aircraft loading and with the engagement air speed and, in fact, may be neutral or negative, thereby causing a lag in aircraft response. A lag in gaining altitude not only increases pilot workload, since a valuable g-response cue is lacking, but also can be dangerous.